Cryptococcus neoformans causes fatal cryptococcal meningoencephalitis and is responsible for 15% of deaths in AIDS patients. This fungus can also cause meningoencephalitis in individuals without apparent immuno-deficiency and such incidences are increasing. The outcome of the current antifungal therapy for patients with cryptococcal meningoencephalitis is far from acceptable, with mortality rates ranging from 10 to 70%. The challenges of treating this disease motivate us to understand cryptococcal pathogenesis and to identify cryptococcal factors that shape the interactions between this pathogen and a mammalian host. Our central premise is that cryptococcal factors that modulate host response or neurotropism could provide targets for antifungal therapy, diagnosis, or prevention. Glycosylphosphatidylinositol (GPI)-modified glycoproteins in various eukaryotic pathogens are prominent players in modulating host responses. The effect of these glycoproteins to the host can be protective or deleterious. In C. neoformans, mannoproteins are considered the primary components recognized by the host anti-cryptococcal cell-mediated immune response. However, only several GPI-anchored mannoproteins have been studied in this pathogen, including virulence factors chitin deacetylases Cda1-3 and phospholipase Plb1. We previously found that cryptococcal cells overexpressing an anti-virulence transcription factor Znf2 can confer rare sterilizing immunity to the host against a subsequent challenge by an otherwise lethal wild type strain. Remarkably, 25 out of the 49 predicted GPI-anchored mannoprotein genes in the genome of C. neoformans H99 are regulated by Znf2, with 10 being upregulated and 15 downregulated. This indicates the importance of Znf2 in controlling mannoprotein presentation in this fungus. A recent study indicates that 13 mannoprotein genes are highly differentially expressed in lungs of both mice and monkeys during cryptococcal infection. Given that GPI-anchored mannoproteins are the major class of cell wall proteins and they are also components of the capsule in C. neoformans, we hypothesize that these proteins represent one of the most important groups of factors modulating cryptococcal interactions with the host. Here we will systematically examine the role of these 49 GPI-anchored mannoproteins in modulating Cryptococcus-host interactions. To achieve this goal, we propose to complete a gene deletion and gene overexpression library for the GPI-anchored mannoproteins. We will use these mutants to define the roles of these mannoproteins in cryptococcal interaction with the host in an animal model of cryptococcosis. Successful completion of the proposed work will reveal cryptococcal factors that can be exploited in the future by us or others to investigate host immunity, antifungal targets, or diagnostic markers. The exploratory nature of the proposed work and the potential impact it has on our understanding of the interaction between the host and this deadly fungus fits perfectly for the R21 mechanism.